Accelerated setting, that is the accelerated binding and/or hardening of hydraulic cements in general and Portland cement in particular, may be obtained by means of external heating or by the addition of an accelerator to the concrete mix. Since the equipment required and the actual generation process for external heating techniques is expensive and not always feasible for on-the-job conditions, it is common practice in the art to employ chemical additives as accelerators in concrete mix. Generally, calcium chloride is used as an accelerator because of its low cost and effectiveness.
The widespread use of concrete in contact with metal, particularly with the advent of large-scale use of pre-stressed, post-stressed and reinforcing steel in structural concrete, has given rise to corrosion problems. While calcium chloride is effective in accelerating the setting of cement, it is used with reluctance, if at all, in applications where concrete is in contact with metal, such as for example, in reinforced and pre-stressed concrete, because it corrodes the metal which it contacts or which is embedded in the concrete. Chloride ions exhibit corrosiveness towards metals and steel in particular, in the presence of concrete if the quality of the concrete is low, if the steel is improperly embedded, or if the concrete is of the high slump variety. Also, corrosion may result in concrete containing calcium chloride if air or water gain access to the steel through porous concrete or through voids under steel bars or cables embedded in the concrete.
The present invention provides for a class of additives for hydraulic cement which is as effective as calcium chloride in the binding and/or hardening of the cement without the corrosion promoting properties of calcium chloride.